You should be able to run the car with the radiator cap off and when the thermostat opens, it will suck in the coolant and then top it off. Either that or there must be a bleeder screw atop the thermostat housing for bleeding.
Need to know the year and engine size so I can help you. NEUTZ air lock? system blockage? is the thermostat installed correctly? 6.0 Engine Yes on the thermistat installed correctly
You may have installed the thermostat backwards.
Be sure it is installed correctly and/or change brands.
A thermostat that is stuck in the closed position is a very common cause of overheating. A new thermostat should be installed every time the coolant is changed.
Would need to know if car was overheating before waterpump and thermostat was replaced.
If the thermostat was not installed correctly, the housing can crack but it would leak engine coolant not oil.
Make sure the thermostat is installed correctly, then there are other factors to consider. Your radiator may be plugged or corroded ( if you run anything but the correct mix of coolant and water you will have this), The radiator fans may not be engaging, the water pump may be bad, you could have a head gasket failure.
Follow the big hose from the top of the radiator to the engine block. You should find a removable housing that contains the thermostat. After you remove this, remember which way the thermostat is installed so you replace it correctly.
If you look at the bottom of the thermostat you will see the spring wrapped around the valve that goes up & down to open & close the thermostat. The spring is made from heat sensitive metal and is what opens and closes the thermostat as it expands & contracts. If the thermostat is installed upside down, the spring will not be in the coolant and will not open until it gets hotter than the specified temp of the thermostat and could cause overheating problems and coolant flow issues.
You have to drain the radiator then remove the two bolts on the top flange of the top hose and under that is the thermostat. See how it is installed so you put the replacement one in correctly.
They can be safer, as long as they are installed correctly. They don't need a brick chimney for venting, they also can be installed near wood because they are insulated to prevent overheating. They are just as safe, if not safer, as brick installed fireplaces.
The thermostat might be installed backwards. The thermostat will still work, but not as efficiently as if it were installed properly. It sounds like engine is still not receiving proper water flow. This is a common mistake.